Method of coating sintered hard metal bodies and hard metal body coated according to the method

ABSTRACT

THE INVENTION RESIDES IN AN IMPROVED METHOD OF APPLYING TO A SURFACE OF A SINTERED HARD METAL BODY A THIN LAYER OF AN EXTREMELY HARD AND WEAR-RESISTANT METALLIC CARBIDE. THE METHOD CONSISTS IN HEATING THE BODY TO AN ELEVATED TEMPERATURE IN AN ATMOSPHERE OF A GAS MIXTURE COMPOSED OF HYDROGEN, A GASEOUS COMPOUND OF A GAS MIXTURE COMPOUND WHICH SAID METALLIC CARBIDES IS TO BE FORMED AND AN AMOUNT OF A GASEOUS HYDROCARBON SO RESTRICTED AS TO BE UNABLE TO SUPPLY A MOJOR PROPORTION OF THE CARBON NECESSARY FOR THE DEVELOPMENT OF SAID METALLIC CARBIDE.

United States Patent 3,832,221 METHOD OF COATING SINTERED HARD METAL BODIES AND HARD METAL BODY COATED ACCORDING TO THE METHOD Carl Sven Gustaf Ekemar, Lannersta, Sweden, assignor to Sandco Limited, Ottawa, Canada No Drawing. Filed Feb. 13, 1970, Ser. No. 11,294 Claims priority, application Sweden, Feb. 21, 1969,

Int. 01: C23c 11/00 US. Cl. 117-106 1 Claim ABSTRACT OF THE DISCLOSURE The present invention relates to a method of coating sintered hard metal bodies with thin layers of extremely hard and wear-resistant metallic carbide.

It is previously known, that surface layers consisting of carbides of one or more of the elements belonging to the 3rd up to and including the 6th groups of the periodical system can be applied on metallic and other materials. Such layers, for instance consisting of TiC, may be applied on a metallic support or substratum by deposition, at an elevated temperature, from a gaseous phase containing metal halide, hydrocarbon and hydrogen.

The above-mentioned process may, for instance, employ a gaseous phase comprising titanium tetrachloride, methane and hydrogen, in which case, the following reactions are important:

(1) TiCl +CH =TiC+4HCl 2) CH =C+2H In order to avoid precipitation of free carbon together with the carbides it is customary to use gas-mixtures which do not contain a greater part of volatile hydrocarbon compounds than corresponds to the equilibrium of the reaction (2) at the precipitation temperature, and in which not more, but preferably as much metal halide is included, as is equivalent with the volatile hydrocarbons (reaction 1).

It has been found that among the materials which are suitable for such deposition and thereby can obtain considerable improvements of the qualities as a substantially increased hardness and wear-resistance, are also included sintered hard metal, which in the first place refers to the kind of material which, except for a content of binder metal like Co, Ni and/or Fe, consists essentially of one or more hard metal carbides of preferably the metals W, Ti, Ta, Nb, Mo, Cr, V, Zr and/or Hf. These materials may moreover contain hard borides, silicides and/or nitrides, for instance of the enumerated metals.

The reactions which occur in the formation of carbide layers on hard metals as substrata have, however, not been closely investigated heretofore, and it had been assumed schematically that the process in general ought to follow the mentioned type-reaction, comprising gaseous metal halide and hydrogen and also hydrocarbon, for in stance methane, as a carbon-emitting medium. The possibility of producing a carbide layer, by reactions in which the hard metal substratum itself has been able to emit the necessary amount of carbon during the process, has

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been considered as excluded. The reason for this opinion was the expericence from coating of steel, showing that steels containing elements which are normally included in hard metal such as W, V, Mo and others, and which at the same time had a high carbon content, were not able to emit carbon in forming surface-coatings of TiC, because the carbon was firmly bound in the form of carbides with the mentioned elements of the steel.

In practical use of carbide-coated hard metal bodies for instance in the field of chip-cutting machining, there have been obtained, until now, very uneven results, which has been a serious disadvantage.

Surprisingly, it has now been found that, under certain circumstances, it is possible to produce on a hard metal substratum a carbide surface layer with excellent qualities in several regards provided the reacting gas mixture during the carbide-forming process is so composed that its carbon content (for instance, bound in the form of hydrocarbons) is essentially below the necessary content for carbide formation. For ensuring this it has been found advantageous for the gas to be so composed that it emits at the most 40%, and preferably not more than 20%, of the amount of carbon required for the carbide formation. This means that the substratum itself has to emit the greater part of the carbon which is necessary for the carbide formation. In this case the reaction can be illustrated by the chemical formula:

It has, thus, been possible to adapt and change the deposition process mentioned before (which broadly is wellknown) so that the chemical transformation mainly proceeds according to reaction 3), although from a thermodynamic point of view this may seem less favourable than reaction 1). The same has been made possible by a suitable adjustment of the amounts of hydrogen and metal halide at the same time as the before-mentioned maintenance of a carbon deficit in the gaseous phase. An addition of gaseous hydrocarbon to the gas mixture entering the reaction vessel should generally be avoided, but in any case it should not exceed 1% by volume and preferably 0.5% by volume of the gas-mixture used in the process. Under the circumstances under which the reaction normally occurs-i.e. a temperature between 700- 1100 C. and a gas pressure between 1-100 mm. Hg--the volume of metal halide must be 0.01-0.04 of the amount of hydrogen, preferably more than 0.02 and less than 0.03. The reaction time, the exact length of which is determined by the mentioned procedural variables, is normally 1-6 hours depending on the desired thickness of the deposit of surfacing layer. In general one employs a reaction time which exceeds 2 hours and preferably at the most lasts up to 5 hours.

Although the carbon in the hard metal substratum is very firmly bound in the form of carbides of metals like W, Ta, Nb, Ti and others, it is possible to regulate the process so that a thin intermediate layer of the hard metal is decarburized. This carbon diffuses to the surface and reacts with the metal halide in question and hydrogen, forming the carbide layer. It has been found very important for obtaining more uniform and better results in the practical exploitation of the process that the mentioned decarburized intermediate layer has a definite composition and extension.

This has been made possible by the before-mentioned determining of the process 'variables and also by determining the conditions of the composition which the hard metal support or the substratum have to fulfill.

Hard substances, which have been found to be suitably active to act as carbon emitters, as for instance carbides of W, Mo and/ or Cr, should be included at contents of at least by volume, preferably over 35% by volume, WC preferably being included as a component.

The binder metal normally consists of Co, Ni and/or Fe, preferably Co, and should be included in the substratum in contents of between 2-30% by volume, preferably more than 5% and at most 25% by volume, spe cifically between and by volume.

The above by volume analysis of the substrate can be expressed in percentages by weight as follows:

(1) carbides of W, Mo and/or Cr at least 15% by weight, (2) up to 20% by weight Co, Ni and/or Fe, (3) and the rest carbides of Ti, Zr, Hf, Ta, Nb and V.

Other hard substances, which have been found too stable to emit carbon in any appreciable degree, as for instance carbides of Ti, Zr, Hf, Ta, Nb, and V, i.e. mainly carbides with so-called cubic crystal structure, may constitute the remainder.

The composition of the mentioned intermediate layer will to an essential extent correspond to certain low carbon phases in the three component system W-Co-C, in the first place so called -phase, usually written Co W C, a structural constituent generally well known in the cemented carbide field. It has often been found advantageous to substitute for Co in the mentioned phases entirely or partly Fe, Ni, and/ or Cr.

In general, a noticeable decarburization occurs to a depth of at least 20 m. beneath the surface of the substratum, while the distinctly marked and relatively uniformly developed intermediate layer--which has an essential part of low carbon phaseshas a thickness of the order of l-l2 am. Preferably, however, the thickness of the layer should exceed 3 ,um. and be below 10 #m. in most cases.

Among the factors which contribute to the improved properties of the product coated according to the invention may be pointed out the levelling of unfavourable differences in quality inside the substratum and between the substratum and carbide layer-for instance, with regard to thermal expansion and mechanical strength.

A further advantage is the exceptionally good bond between substratum and layer (of so-called metallurgical type) which results when the above-mentioned low carbon phases are formed and grow simultaneously with the titanium carbide layer on the hard metal.

Another essential advantage is the remarkable increase of hardness due to the decarburization in the substratum immediately below the carbide surface 1ayerwhereby there is realized, among other things, an increased resistance to plastic deformation on the part of the coated hard metal body.

The precipitated surfacing layer, which should be very thin (generally, between l-12 ,um. and often about 26 pun.) consists of extremely fine grained carbide, for instance TiC. NbC and/ or TaC.

It has been found very advantageous if the carbide layer is so built that it contains a particularly fine-grained layer next to the substratum and a somewhat coarser grained growth zone at the surface. The mean grain size of the inner layer should be about 0.02-0.15 ,um., and that of the growth zone about 0.2-0.4 ,urn. As one example there may be mentioned a coated body wherein the mean grain size of the inner layer was about 0.1 um. and that of the growth zone was about 0.3 m.

Products manufactured according to the invention are in practical use especially characterized by a very good and uniform quality. Among the reasons for this may be mentioned the improved adherence between layer and substratum; also, the increased resistance against thermal shocks and mechanical impact stresses; and better resistance of the substrate of the layer against plastic deformation.

Among the fields of application where coated hard metal according to the invention has shown superior qualities compared to conventional hard metal may be mentioned tools and inserts forchip cutting machining; inserts and wear parts in rock drilling; coldand hot-forming tools including tools for wire, strip or tube drawing; shearing and pressing tools; sealing means for pumps and the like; and wear parts of different kinds.

. In a wider sense the quality-improving efiect whicha carbide coating has upon a hard metal body is based on metallurgical laws of rather complex nature. In certain fields of use-for example, chip forming machiningthe favorable effect which may be obtained by an addition of TiC to normal hard metal of WC-Co-type is well known. TiC and other carbides contribute to a decreased speed of the diffusion-caused wear reactions between hard metal insert and work piece during the cutting operation. One advantage of the invention is, therefore, that the TiC layer deposited on the hard metal body serves as a so called diffusion barrier, which barrier prevents harmful wear reactions. At the same time, the hard metal body maintains its original excellent strength properties.

The following example illustrates the conditions under which sintered hard metal bodies have been coated with TiC according to the invention.

Example 1 The coating took place in a retort, i.e., a reaction vessel of a type known in itself, permitting evacuation, a through flow of gas, and heating up to high temperature.

Substratum: 1000 pieces of precision-ground hard metal inserts intended for cutting. The hard metal grade (type 8,) contained about 40% by volume WC, 15 by volume Co and 45% by volume cubic carbides in the form of TiC, TaC and NbC.

Coating: The hard metal inserts were placed in the retort, whereupon a gas with the composition (in percent by volume): 97 H 2.5 TiCl, and 0.5 CH was passed through the charge. The gas pressure was 20 mm. Hg and the incoming gas flow was 760 N cmfi/min. The reaction took place at a temperature of 875 C., and the coating obtained the desired thickness after 2.5 hours.

Results of the coating: Metallographic and physical examinations showed, among other things, that the surface layer of TiC was uniformly developed with a compact, pore-free structure and a thickness of about 4 ,ul'll. Nearest the substratum was observed an inner more :fine grained zone with a grain size of about 0.09 ,um. The outer zone had a grain size of about 0.3 ,um. The intermediate layer, bodering on the TiC-layer on the substratum, showed a uniformly decarburized zone with a thickness of about 7 ,um. W-Co-C-phases that were present consisted mainly of (eta) -phase, but also some other phases were observed.

The following example 2 shows the great improvment which can be obtained by the invention. Here, comparison is being made in the field of cutting between inserts coated with a TiC-layer according to the invention, inserts coated with a layer according to the method wherein the necessary amount of carbon for the carbide formation is added mainly by means of gaseous hydrocarbon (S with a non-uniform layer according to the example), and normal uncoated inserts.

Example 2 Crater Wear de th Time Hard metal grade mm: 2m: min:

S4 (with layer according to the invention). 0. 09 5 15 SIP 0.20 56 15 0. 10 7 15 Notice: in several different operations demanding high toughness the coated S inserts showed the same strength as the uncoated inserts respectively the inserts of grade 8, coated with a non-uniform layer.

Composition, in percent by weight, of the hard metal:

Grade Ni Mo TiC TaC NbC C 0; WC

S4 9.5 11.9 6 4 Remainder SIP 9.5 19 12.2 3.8 Do. F02 10 2 60 3 2 5 Do.

It is clear, from this example, that the resistance to flank wear has in broad outline increased times by applying the improved carbide surface layer, and that an enormous decrease in cratering has been eifected.

Inserts of grade S with maintained toughness have by means of a coating according to the invention, reached the same wear-resistance as corresponding tools in the extremely wear-resistant but at the same time considerably less tenacious grade F0 I claim:

1. In a method of coating a surface of a sintered hard metal body, consisting essentially of (a) at least one member of the group consisting of the carbides of W, Mo and Cr, at least 10% by volume; up to 30% by volume of (b) binder metal selected from the group consisting of Co, Ni and Fe, and the rest (0) at least one member of the group consisting of T iC, ZrC, HfC, TaC, NbC and VC, with a thin and extremely hard and wearresistant carbide layer consisting of at least one hard metallic carbide selected from the group consisting of HC, NbC, TaC, ZrC, HfC, VC and WC, by conducting a gas mixture at high temperature over said sintered hard metal body, said gas mixture being intended to form the carbide coating layer and consisting essentially of a gaseous metal halide of the metal or metals for forming said carbide coating layer together with hydrogen and a gaseous hydrocarbon, the improvement which consists in so composing the gas mixture that the ratio of the volumes of metal halide and hydrogen are in the range 0.01-0.04 to 1 and that its content of carbon in the form of gaseous hydrocarbon, not more than 1 percent by volume of said gas mixture, is not more than 20% by weight of the carbon necessary for the formation of the desired carbide of said carbide coating layer.

References Cited UNITED STATES PATENTS 3,393,084 7/1968 Hartwig 117-106 C 3,368,914 2/1968 Darnell et a1 117106 C 3,108,013 10/1963 Pao Jen Chao et a1.

117-107.2 P 3,306,764 2/1967 Lews et al 117-406 C 3,348,967 10/1967 Hucke 117118 3,166,614 1/1965 Taylor 117-406 CB OTHER REFERENCES The Chemical Vapor Deposition of Titanium Carbide Coatings on Iron; J. Electrochem. $06., 12/ 67, pp. 1230- 1235.

DANIEL J. FRITSCH, Primary Examiner US. Cl. X.R. 117-118, 169 R 

